The present invention relates to semiconductor devices, liquid crystal display devices and electronic equipment and relates, in particular, to a semiconductor device whose circuit blocks are constructed of transistor groups where individual transistors have variations in an on-state current and an off-state current, and a liquid crystal display device and electronic equipment, which employ the semiconductor device.
Lately, there is a liquid crystal display device equipped with a semiconductor circuit constructed of transistors formed on a glass substrate (refer to, for example, JP H04-195123 A) as electronic equipment that employs a semiconductor device. Moreover, a circuit including transistors and so on will presumably be formed also on a flexible substrate such as plastics substrate, which can be processed by a low temperature process in the future.
The transistors formed on a glass substrate or a plastics substrate as described above have variations in the on-state current and the off-state current larger than those of the transistors formed on a silicon substrate, causing a problem that the product yield is reduced. Power consumption increases when, for example, the on-state current is excessively large, while the driving abilities of the transistors become insufficient and the circuit does sometimes not correctly operate when the on-state current is excessively small. Moreover, a circuit design balance is lost in either case, resulting in reducing the operation margin. Otherwise, when the off-state current is excessively large, a standby current increases or the signals and electric charges leak, resulting in a fail in holding data or incorrect circuit operation.
As a typical solution approach to the conventional transistor defects as described above, there is a semiconductor device whose transistors are connected in series or connected in parallel.
However, the conventional semiconductor device employing the way of connecting the transistors in series is effective for the off-state current failure because the current can be turned off when either one of the transistors is normal, but is inappropriate for the on-state current failure because the desired current does not flow when either one of the transistors suffers an on-state current failure and particularly when the current is small. Moreover, the semiconductor device employing the way of connecting the transistors in parallel is effective for the failure of a small on-state current because a normal current flows when either one of the transistors is normal particularly, but is inappropriate for the off-state current failure because the current cannot be turned off when either one of the transistors is defective.